Critics point out that at six per cent of voter intentions in round one of the elections, Mr Bayrou stood no chance of election.

But if his followers were to heed his call, the extra votes could push Mr Macron ahead of François Fillon, conservative candidate for Les Républicans party and his main rival.

Recent polls have placed the pair neck and neck in the knockout first round on April 23, in which far-Right Front National candidate Marine Le Pen is expected to come first.

Either candidate is expected to go on to win the May 7 run-off against Ms Le Pen, whose is nevertheless closing the gap.

One poll this week suggested Mr Macron had lost ground to Mr Fillon, whose popularity nosedived following allegations he had his British wife paid more than €900,000 for a "fake job" as his parliamentary assistant.

Mr Bayrou said he would hold talks in the coming hours after setting several conditions for the alliance, including creating a law to "moralise" French politics - a dig at Mr Fillon and Ms Le Pen, who also faces corruption allegations.

The centrist stalwart has previously been scathing of Mr Macron, calling the ex-Rothschild banker too close to "the forces of money".

But describing French democracy as under threat, he slammed Mr Fillon's camp as happy to "tacitly accept" the "very serious" corruption allegations against him that he said were damaging France's image abroad.

The Socialists, meanwhile, who he backed in 2012, were in disarray, a situation that could pave the way for the far-Right, he claimed.

"What's at stake with this alliance is to restore hope," Mr Bayrou said.

Observers said that Mr Bayrou, a former school teacher who was long aligned to the centre-Right, could lend the politically inexperienced and urbane Mr Macron gravitas and a line of communication with provincial France. Mr Bayrou is mayor of Pau in the French Pyrenees and has previously professed a love of tractors.

But rivals welcomed the political coup de theatre.

Luc Chatel, a Fillon ally, said that Mr Bayrou would steal no votes from conservatives as he had already "betrayed the Right-wing electorate" by calling on his supporters not to vote for Nicolas Sarkozy in 2012. Many feel this lost Mr Sarkozy the election.

Florian Philippot, Front National number two, said that joining forces with an 'old-timer" like Mr Bayrou put paid to Mr Macron's claims to want to modernise French politics.

The FN had its own problems on Wednesday as Ms Le Pen's chief of staff was placed under formal investigation as part of a probe into alleged misuse of EU funds to pay parliamentary assistants. Catherine Griset had earlier been questioned by judicial police along with Ms Le Pen's bodyguard, Thierry Legier, who was later released.

Ms Le Pen, who denies any wrongdoing, dismissed the setback as "political maneuvering".

"The French can tell the difference between genuine scandals and political dirty-tricks," she said.